What is the difference between map and flatMap in Java streams?

Distinguishes one-to-one transformation from transformation plus flattening for nested collections, streams, and optionals.

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TL;DR

map transforms each input element into one output value, so a Stream<Order> can become a Stream<String> of order identifiers. If the mapping function returns another stream or collection-like value, map preserves that nesting. flatMap transforms each input into a stream and then flattens those streams into one stream. For example, orders mapped to their line-item streams become one Stream<LineItem>.

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Data-shape reasoning, nested results, flattening, absence, and readable stream transformations.

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Core answer
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Overview
The key question is what shape each input element becomes after the transformation.
map transforms each input element into one output value, so a Stream<Order> can become a Stream<String> of order identifiers. If the mapping function returns another stream or collection-like value, map preserves that nesting. flatMap transforms each input into a stream and then flattens those streams into one stream. For example, orders mapped to their line-item streams become one Stream<LineItem>.
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Details

Using flatMap whenever a lambda looks complex.
Leaving nested streams or optionals unintentionally.
Flattening away parent context that later logic needs.
Ignoring the exact mapper return type.

What is the difference between map and flatMap in Java streams?

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